JPH1075528A - Current limiter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a current limiter which includes a superconducting element which can be connected to an electric circuit and has a critical current density. SOLUTION: A current limiter includes a 1st superconducting element 4 which can be connected to an electric circuit. The superconducting element 4 has a critical current density. The start of a defect state is detected by a 2nd superconducting element 12. In response to the 2nd superconducting element 12, a relation between the critical current density and a current produced in the 1st superconducting element 4 is changed by a coil 13. By this change, the state of the 1st superconducting element is transferred to a resistive state.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention can be connected to an electric circuit,
It relates to a current limiting device of the type comprising a superconducting element having a critical current density.

[0002]

Problems to be solved by the prior art and the invention
A current limiting device of the type known as a fault current limiter (FCL) has recently been proposed, some examples of which are described in our WO-A-96 / 30990. In a typical example, a length of high temperature (HTc) superconductor is installed in a circuit that carries current. HTc materials have a relatively high (typically equal to liquid nitrogen temperature) critical temperature and a critical current (strictly, current density) that varies inversely with the applied magnetic field. If the current carried by this superconductor exceeds the critical current,
The superconductor material transitions into a resistive state and acts to limit the current being carried. The critical current value at which this transition occurs can be changed by changing the applied magnetic field. In WO-A-96 / 30990, Applicants describe a method of resetting a current limiting device after a superconductor has transitioned to a resistive state.

[0003] Applicants have studied the professional theory that a superconducting element converts from its superconducting state to its resistive state.
Applicants believe that due to the thermal conductivity constraints of the superconducting element, the change is gradual along the length of the superconducting element, and in some cases very high power densities occur in the superconducting element. Discovered that it could cause the device to fail. The analysis of the present applicant is described below. The following symbols are used as typical values: ρ Electrical resistivity 10 ^-6 Ω-m K Thermal conductivity 0.5 Wm ^-1 K ^-1 C _p Specific heat 150 Jkg ^-1 1K ^-1 γ Density 6300 kg m ^-3 h Heat transfer to coolant 200 Wm ^-2 K ^-1 θ _o Coolant temperature 77 K θ _c Superconducting critical temperature 90 K θ _max Maximum safe temperature 800 K θ Temperature variables J, J _c Current density, critical current density δ Superconducting conversion width I _max, If trip current, limiting current R ₀ Nominal normal-state resistance L, A length, cross-sectional area For “series resistor” type FCL system: I _max = J _c A (peak) I _f = V / (ρL / A) I _f / I _max = V / ρJ _c L Thermal diffusivity is m = K / γC _p 5.3 10 ^-7 ms ^-1 This means that at 0.01 s, the thermal disturbance is only 5 nanometers It means moving. Therefore, we can ignore the AC heating effect of the supply current and use the appropriate RMS value in the heat calculation.

[0004] Considering an HTC element consisting of a slab or film cooled on one side, under normal conditions, the temperature distribution is given by: K (∂ ² θ) / (∂x ² )- γC _p (∂θ) / (∂t)
= J ² ρ / 2 x = 0, the heat flow is Q = h (θ−θ ₀ ). When t = 0, θ = θ ₀ everywhere. This was solved numerically for current densities of 10 ⁸ , 10 ⁹ Am ^-1 . The temperature distribution across a 200 micron thick film is shown in FIGS. As is clear here,
(Film boiling) Heat transfer to nitrogen is much lower than the rate of heat release. Therefore, for most purposes, the system can be considered adiabatic.

Burnout Protection Consider an FCL consisting essentially of a high temperature superconducting element in series with the line (or, in the case of the shielded inductor type, a high temperature superconducting element inductively connected to the line current). This element is of sufficient length that, under normal conditions, its resistance is high enough to limit the current flowing therethrough to a low value. In the superconducting state, the current is reduced to the value I without dissipation of power.
c. In fact, this superconducting element is not uniform and some parts will have a lower critical current density than others. In the event of a fault, for example in the case of a short circuit, the current will tend to exceed Ic, the rate of rise of which depends on the system inductance or on the part of the alternating current cycle in which the fault has occurred. Initially,
The element will have approximately sufficient resistance to generate a resistance that limits the current to the waterfront.

If, instead of a short circuit, the fault causes a small overload, or if the fault current rises relatively slowly, some of the superconductor will be lost due to the non-uniformity of the elements. Normal, but to a large extent, will be superconductive. As mentioned above, the propagation speed is very low,
At a time comparable to the AC current period, the resistance region does not grow. The resistance value follows as the current increases,
"Latches" to its highest value when current drops in that cycle. These effects can be modeled as follows: Here we assume that the critical current has a normal distribution:

[0007]

(Equation 1) FIG. 3 shows J _cO = 10 ⁸ A / m ² , δ = 10 ⁷ A / m ² sized to have R ₀ = 100Ω and I _max = 1000A.
5 shows the V / I characteristics of the elements with suffixes. When the load resistance value is r and the system voltage is V ₀ ,

[0008]

(Equation 2) This equation can be solved graphically for I. V
₀ = 24 kV and 1Ω (almost short-circuited) to 20Ω
Representative plots of I versus time for various values of r up to (intermediate overload) and 40Ω (normal) are shown in FIG. Here, as it is apparent, the current is limited normal trip value, just below the I _max. The corresponding resistance values are shown in FIG. Therefore, as it is,
An FCL system will limit the current to approximately its critical current value. Unfortunately, this is typically very large for resistive conductors and will exhibit very high power densities.

The time required to reach the maximum safe temperature is

[0010]

(Equation 3) This is 140 ms for J = 10 ⁸ Am ⁻² and 1.4 ms for J = 10 ⁹ Am ⁻² . This means that at 10 ⁹ Am ^-2 , the superconductor can be destroyed in less than one cycle of the supply current. The protection problem is that when a fault causes a very light overload, resulting in resistance in only a small area,
It is likely to be the toughest. This resistance is so small that it is difficult to detect. When limiting a small resistive region, one must wait until it has grown to a detectable level. Broom and Rhoderick (Brit J App Phys, 11, 292, 196
According to (0), the rate at which the normal region propagates is given by the following equation.

[0011]

(Equation 4) This is 0.014 ms-1 at J = 10 ⁸ Am ^-2 and J = 10 ⁹ Am ^-2
Is 0.14 ms ^-1 . This value is much lower than that of the well-known low-temperature superconductor. In the time to reach the maximum safe temperature, the length of the resistive zone, its resistance and the voltage across it increase as follows:

[0012]

(Equation 5) This is about 200 mV.

[0013]

SUMMARY OF THE INVENTION According to the present invention, a current limiting device can be connected to an electrical circuit, and includes a first superconducting element having a critical current density and an onset of a fault condition. A detecting means for detecting, and a changing means for changing a relation between a critical current density and a current generated in the first superconducting element in response to the detecting means and converting the superconducting element into a resistance state are included. According to the present invention, the speed at which the first superconducting element converts to a resistive state under a fault condition to prevent the above-described overload and overheating problems is increased. In one approach, a magnetic field is applied at the onset of a fault condition. Therefore,
The altering means includes a magnetic field generator that generates a magnetic field to reduce the critical current density of the first superconducting element. The magnetic field generator typically includes a solenoid.

While the onset of a fault condition can be detected by sensing current in an electrical circuit using known techniques, the fault condition is detected by sensing the occurrence of resistivity in a superconducting element. Is preferred. To detect the occurrence of resistivity and activate the magnetic field generator, typically, 1
A resistance value on the order of 00 micro ohms is detected. The occurrence of resistivity can be detected in two ways. In a first alternative, this monitors the voltage across the first superconducting element,
When the voltage exceeds a predetermined threshold, the change can be performed by activating a change unit, for example, a magnetic field generator. The stray inductance can generate a voltage along the entire length of the first superconducting element that masks any voltage due to electrical resistivity. This measures the voltage at several points along the first superconducting element,
It is overcome by effectively dividing the superconducting element into several sections. The potential difference across each section is then measured. Preferably, the sections are of a predetermined length and the voltage generated by the stray inductance across each section is approximately the same. For example, if the superconducting elements are symmetric, each section will be of equal length. Assuming no other voltage sources in the element, the potential difference across each section will be about the same. However, if any section of the element becomes resistive, the potential difference across that section will be different from that of the other sections, so that a fault condition can be detected. If sections of the required length are not available, the different effects of stray inductance across each section can be considered using computer analysis as would be known to one skilled in the art.

[0015] In a second alternative, the occurrence of resistivity can be detected by injecting a high frequency sampling current into the first superconducting element. The frequency of the sampling current or the probe current is considerably different from the line current. The voltage across the element is monitored and rectified by a phase detection detector in close contact with the probe current. The signal thus produced does not include any derivations from the line current. The component from the stray inductance is constant and can therefore be easily subtracted, leaving a signal representing the resistance of the element. When using a magnetic field to activate a superconducting element, the L / R constant of the magnetic field coil is determined by the speed at which the magnetic field can be switched,
As a result, it will give a limit of the maximum value J _c that can be used in the region of 10 ⁸ Am ^-2 typically.

In another example, a capacitor is connected in series with the magnetic field generator via a switch, and the detecting means includes means for closing the switch when a fault condition is detected. In this case, the switch may include a thyratron or a trigger spark gap. In this configuration, the superconducting element is typically driven with a short pulse current by discharging a capacitor. In a further preferred example, the modifying means is a second superconducting element in series with the first superconducting element, the second superconducting element having a critical current density lower than the critical current density of the first superconducting element. A series connection of a coil and a capacitor connected electrically in parallel across the second superconducting element, wherein the coil is physically disposed around the first and second superconducting elements. And a capacitor combination.

The advantage of this arrangement is that it provides a fail-safe mechanism and has few, if any, active components. In another approach, the means of change is:
It includes a capacitor connected to the superconducting element via the switch, and means for closing the switch upon detecting the onset of a fault condition, such that the resulting current through the superconducting element can exceed the critical current of the element. In this approach, instead of changing the critical current density, the current through the element is rapidly driven to a high level, causing a transition to a resistive state.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In all the examples to be described, the superconducting elements will be shown without reference to the low temperature physics generally required to achieve the superconducting state. In the first example shown in FIG. 6, the circuit 1 to be protected comprises a load (Z) 2 and a power supply 3 connected in series with a superconducting element 4 of HTc material.
Surrounding the superconducting element 4 is a solenoid 5 which is connected to another circuit including a capacitor 6, a power supply 7, and a switch 8 (eg, a thyratron). The onset of a fault is detected using the controller 9. The controller 9 operates by sensing the current in the circuit using known techniques, or by detecting the occurrence of resistivity of the superconducting element. Resistivity detection can also protect against damage caused by refrigeration failures and can be performed by monitoring the voltage across superconducting element 4. If this voltage exceeds a predetermined threshold, it is determined that a failure has been detected, and the switch 8 is closed.

This method can be affected by stray inductance in the superconducting element 4. This stray inductance produces a sufficiently large voltage over the entire length of the element, which can mask any voltage due to the occurrence of resistivity. This can be solved by measuring the voltage at several points along the superconducting element, as shown in FIG. Points 20-2
By measuring the voltage at 4, the element is effectively turned into four equal sections 20-21, 21-22, 22-2
3, 23-24, and the potential difference across each of these sections can be calculated. Assuming that the superconducting element is symmetric, the voltage created by the stray inductance in each section of the element will be approximately the same. However, if resistance occurs in any section of the element, the potential difference in that section will be different from the potential difference in other sections.

In the case shown in FIG. 17, if the element has no resistance value, V _20-21 = V _21-22 = V _22-23 = V _23-24 However, there is a resistance value in section _BC. In addition, there is a potential drop due to a resistance that cancels a voltage rise due to inductance. In this case: V _20-21 = V _21-22 = V _22-23 > V _23-24 Therefore, the superconducting element is divided into several sections,
By measuring the potential difference before and after each section, the effects of stray inductance can be identified. The controller 9 measures the voltage at each of the points 20 to 24, calculates the difference _{V 20-2 1 = V 21-22 = V} 22-23 = V 23-24. These potential differences are compared, and if the potential difference in one section is different from the potential difference in another section, the onset of a fault condition is detected and switch 8 is used to activate the magnetic field generator.

As a more sensitive method, an AC sampling current or a probe current is injected into the superconducting element 4. In this case, the frequency of the alternating current is significantly different from the frequency of the line current. A typical detector configuration is shown in FIG. Here, the sampling current is AC
Supplied by applying a voltage across superconducting element 4 using current source 20. The voltage across the superconducting element 4 is filtered by a suitable filter 21, amplified by an operational amplifier 22, and rectified by a phase detector 23 in close contact with the probe current. The resulting signal does not contain any derivative of the line current, and the component from the stray inductance is constant and easily subtractable. The remaining signal represents the resistance of the superconducting element 4, which can be used to activate the switch 8.

To explain the operation of this circuit, consider here the design of a superconducting element (FCL) 4 having the following specifications: Line voltage V ₀ 24 kV Rated normal current I 650 A Trip normal current I _max 2. Assume that the performance of the 1000A fault current _If 240A superconductor is as follows: Critical current density J _c 10 ⁸ A / m ² Transition width 10 ⁷ A / m ² where the rated current is lower than the trip current. Note that almost the entire element is in the superconducting state at this level, chosen 5δ.

The required cross-sectional area of the superconductor is 10 ⁻⁵ m ² , for example, a ceramic substrate, 100 mm
It can take the form of a 100 μ thick film over the width. Necessary one
To obtain a resistance of 00Ω, the total length would have to be 1000 m. In order to make the device 4 reasonably compact, as 1000 bars each 1 m long,
They may be connected in series non-dielectrically, suitably supported, and immersed in liquid nitrogen. In the event of a fault, the FCL 4 will initially develop resistance to the extent necessary to limit the current to a peak-to-peak value of about 1000A. The temperature of the resistive region is 530
It will rise at 0K / sec. When a failure is detected,
When the switch 8 is closed, the protection mechanism is activated,
Resistance occurs throughout the element, thus reducing the current to a 240 A peak-to-peak value. If the mechanism detects a failure and requires one full line cycle (20 ms) to become effective, the peak temperature has risen to 183K. Thereafter, the temperature rises at 300 K / sec, which will take about one second to power down. This can be done using a circuit breaker 15.

The protection mechanism is configured to provide a magnetic field that reduces the critical current (in this case, by a factor of at least 4). This requires a magnetic field with a strength of 0.05-0.1 Tesla, which has an inner diameter of 1.5 m and an outer diameter of 1.6.
m, 1.5 m in length. This coil 5 generates a magnetic field of at least 0.1 T across the FCL element 4 at a current density of 3 A / mm ² . In this field, it will have a 4 × 10 ⁴ Joules stored energy (LI ^2/2). To quickly establish a solenoid magnetic field, solenoid 5
Activated by discharging capacitor 8 from a high voltage via switch 8 (eg, a thyratron) or a trigger spark gap. The value of the inductance and capacitance of the coil may give a single (or a small number) magnetic field pulse longer than the line cycle,
Alternatively, one can choose to provide a rapidly oscillating magnetic field that operates at the required value several times the line cycle. The purpose is to obtain a magnetic field and a peak line current at the same time. Here, the magnetic field oscillating at or near the line frequency will be out of phase and may not be able to achieve a resistance transition. Examples of these two options are shown below: ────────────────────────────────── L _coil I _coil R _coil R _switch CV _C HA Ω Ω μF kV 0.08 1000 0.2 0.1 1000 10 0.0008 10000 0.01 0.1 20 100 The coil currents are shown in FIGS. 9 and 10, respectively, and the switches are closed at 0.01 seconds, and for comparison, The line voltage is also shown.

A second example is shown in FIG. in this case,
The protection mechanism includes a capacitor 10 connected in series with the superconducting element 4 and a voltage source 11. Other components with the same reference numerals as shown in FIG. 6 have the same functions as the components in FIG. In this example, the capacitor 1
0 discharges at the onset of a fault condition, driving current through the superconducting element 4 and changing the superconducting element to a resistive state. To do this, the capacitor 6 needs to be charged as follows. V> I _c R Here, when I _c = 1000 A and R = 100Ω, V> I _c R
Should be 100 kV. Since the superconducting element 4 may have a line voltage across it at the moment the capacitor discharges, a total of V = 12
Must be 4 kV.

Considering an FCL with an inductance of 60 μH having a series resistor of 100 Ω, the current pulse reaches 1000 A when C = 4 μF as shown in FIG. The energy associated with this process is only sufficient to raise the temperature of the superconductor by a few degrees. This system is
It has the advantage of only requiring a fairly small high-voltage capacitor, but has the disadvantage that this capacitor and its control must be floating at the line voltage, increasing complexity. Both of the above mechanisms rely on detecting the onset or overcurrent of a resistive transition and forcing the completion of the resistive transition. As such, an active mechanism is required, but its failure may result in heating of the superconducting element.

FIG. 8 shows a preferred arrangement with an additional or second superconducting element 12 (HTC1). In practice, this superconducting element 12 is part of a larger superconducting element and the other part is superconducting element 4 (HTC
2) may be formed. Superconducting element 12
Are connected in series with the superconducting element 4 and in parallel with the coil 13 and the capacitor 14. A coil 13 surrounds both superconducting elements 4,12. The critical current density of superconducting element 12 is chosen to be slightly lower than the critical current density of superconducting element 4. Further, the superconducting element 12
It has a normal state resistance value that need not be greater than V ₀ / I _c . Superconducting element 4 will provide the majority of the resistance when it becomes resistive. When a failure occurs, the HTC 12 will initially become resistive and the voltage across it will drive current to the magnetic coil 13. The series resonant capacitor 14 provides a low impedance to the coil 13 (and thus can generate significant coil current when the fault is overloaded rather than a short circuit, and the HTC
1 does not initially produce its full resistance), and also ensures that the coil current (and its magnetic field) is in phase with the currents of both superconducting elements.

The effect of the magnetic field is to reduce the critical current, resulting in a resistance in HTC2. This limits the fault current. 12 to 16 illustrate the operation expected from this type of mechanism. Coil 13
Has an inductance of 0.08H and generates a magnetic field of 0.1T at 1000A. Capacitor 14 is 127μ
And the series resistance R _ind of the coil 13 is 0.2
Ω. 12 and 13 show the coil current and the total system current in the case of a short-circuit fault, respectively. Short circuit is 50
Occurs in msec. The HTC 112 generates a resistance with a resistance value of 25Ω, and the coil current sharply increases. HTC24 has a full resistance (100Ω) at 75 msec, which will reduce the current to its limit.

FIGS. 14, 15 and 16 show the coil current under various overload conditions. In each case, HTC1
12 first limits the current to I _c , ie,
R _HTC1 = V _o / I _{c -Resistance} occurs to the extent necessary to make -R _load . Voltage available to fill the coil 13, therefore, V ₀ - the I _c R _load. The main effect of a small overload as compared to a short circuit is to extend the delay before resistance develops throughout the system. The impedance of the series resonant circuit ensures that the coil will acquire significant current within the range of R _HTC1 ≧ R _ind . The possibility of lowering R _ind by cooling the coil 13 to liquid nitrogen temperature must not be ignored.

[Brief description of the drawings]

FIG. 1 is a graph showing the temperature distribution across a 200 μ thick film that changes over time for a film having 10 ⁸ Am ⁻² .

FIG. 2 is a graph showing the temperature distribution across a 200 μ thick film that changes over time for a film with 10 ⁹ Am ⁻² .

FIG. 3 shows that J _C0 = 10 ⁸ A / m ² and δ = 10 ⁷ A / m
² shows the V / I characteristics for an element of a size such that R ₀ = 100Ω and I _max = 1000A.

FIG. 4 is a plot of current versus time for various load resistance values.

FIG. 5 is a plot similar to FIG. 4, but
5 is a plot showing corresponding resistance values.

FIG. 6 is a circuit diagram showing an example of the present invention.

FIG. 7 is a circuit diagram showing another example of the present invention.

FIG. 8 is a circuit diagram showing another example of the present invention.

FIG. 9 is a graph of voltage variation versus time for one way of operating the circuit according to FIG. 6;

10 is a graph of voltage variation versus time for another method of operating the circuit according to FIG. 6;

FIG. 11 is a graph of current variation versus time for one method of operating the circuit shown in FIG.

FIG. 12 shows the coil current for a short circuit fault in the circuit of FIG.

FIG. 13 shows the total system current for a short circuit fault in the circuit of FIG.

FIG. 14 shows the coil current in the circuit of FIG. 8 under certain overload conditions.

FIG. 15 shows the coil current in the circuit of FIG. 8 under another overload condition.

FIG. 16 shows the coil current in the circuit of FIG. 8 under yet another overload condition.

FIG. 17 shows a location along a superconducting element that measures voltage to determine a fault condition.

FIG. 18 shows an example configuration used to detect the onset of voltage in a superconducting element.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Circuit 2 ... Load 3 ... Power supply 4 ... Superconducting element 5 ... Solenoid 6 ... Capacitor 7 ... Power supply 8 ... Switch 9 ... Controller

 ──────────────────────────────────────────────────の Continued on Front Page (72) Inventor Peter Henry Gloucestershire GL 154 Jake Ridney Parken de Western Lodge (No Address)

Claims (12)

[Claims] 1. A first superconducting element that can be connected to an electrical circuit and has a critical current density, detection means for detecting the onset of a fault condition, and a critical current density and A current limiting device for changing the relationship with the current generated in the superconducting element and converting the superconducting element into a resistance state. 2. The apparatus of claim 1, wherein the altering means includes a magnetic field generator for generating a magnetic field to reduce a critical current density of the first superconducting element. 3. The apparatus of claim 2, wherein the magnetic field generator includes a solenoid. 4. The apparatus according to claim 2, further comprising a capacitor connected in series with the magnetic field generator via a switch, and means for closing the switch when the detecting means detects a fault condition. An apparatus characterized by including. 5. The device of claim 4, wherein the switch comprises a thyratron or a trigger spark gap. 6. The apparatus of claim 1, wherein the altering means is a second superconducting element in series with the first superconducting element, the critical current density being lower than the critical current density of the first superconducting element. And a coil and a series-connected capacitor combination electrically arranged in parallel across the second superconducting element, wherein the coil is physically located around the first and second superconducting elements. An apparatus comprising: an installed coil / series connected capacitor combination. 7. The apparatus of claim 6, wherein current flows through the first superconducting element at a line frequency, and the coil and series connected capacitor resonate substantially at the line frequency. 8. The apparatus of claim 1 wherein the altering means closes the switch when detecting the onset of a fault condition with the capacitor connected to the first superconducting element via the switch, and the resulting superconducting element. Means for ensuring that the current through the conducting element exceeds the critical current of the superconducting element. 9. The apparatus according to claim 1, wherein the detecting means monitors a voltage across the first superconducting element, and when the monitored voltage exceeds a predetermined threshold. A voltage monitor for activating the changing means. 10. The apparatus of claim 9, wherein the voltage monitor monitors a voltage across multiple sections of the first superconducting element, wherein the monitored voltage across one section exceeds a predetermined threshold, and Activating the altering means if the voltage crossing the section differs by more than a predetermined amount. 11. The device of claim 10, wherein the sections have approximately the same length. 12. The apparatus according to claim 1, wherein the detecting means includes a sampling current source for injecting a sampling current into the first superconducting element, and a first superconducting element for passing the sampling current. Means for determining the resistance of the first superconducting element by monitoring the response of the conducting element, wherein the detecting means activates the changing means when the monitored response indicates the onset of a fault condition. Equipment to do.